The present invention relates to an optical transmission line composed of fusion-splicing optical fibers which have different structures each other; and, in particular, to an optical transmission line including a portion in which optical fibers having mode field diameters different from each other are fusion-spliced.
Optical fibers are connected together by fusion splice, which enables a permanent connection, in order to restrain the splice loss at their splice portion from fluctuating. However, the splice loss at the fusion-splice portion is greater when optical fibers having structures different from each other are fusion-spliced together than when optical fibers having the same structure are fusion-spliced together
For example, there is a case where a dispersion-compensating optical fiber having a negative chromatic dispersion at a wavelength of 1.55 xcexcm is fusion-spliced to a standard single-mode optical fiber having a zero-dispersion wavelength in a 1.3-xcexcm wavelength band and a positive chromatic dispersion at a wavelength of 1.55 xcexcm, so as to carry out dispersion compensation. The single-mode optical fiber and dispersion-compensating optical fiber greatly differ from each other in terms of their fiber structures. Therefore, the splice loss at their fusion-splice portion is about 1.0 to 2.0 dB, which is large.
Constructing an optical transmission line by alternately fusion-splice positive and negative dispersion optical fibers respectively having positive and negative chromatic dispersions at a predetermined wavelength, for example, has also been under consideration. Constructing an optical transmission line as such yields a predetermined value of chromatic dispersion or higher at each point on the optical transmission line, so as to restrain transmission characteristics from deteriorating due to four-wave mixing, and lowers the average chromatic dispersion of the optical transmission line as a whole, so as to restrain transmission characteristics from deteriorating due to the chromatic dispersion. In this case, for example, the positive dispersion optical fiber has a step-index type refractive index profile with a core diameter of 8 xcexcm and a refractive index difference of 0.35%, whereas the negative dispersion optical fiber has a W type refractive index profile, whereby their fiber structures greatly differ from each other. Therefore, the splice loss at their fusion-splice portion is about 0.8 to 1.5 dB, which is large.
Optical fiber connecting methods for eliminating such problems are disclosed in Japanese Patent Application Laid-Open No. HEI 3-130705 and Japanese Patent Application Laid-Open No. SHO 57-24906. In the optical fiber connecting method disclosed in Japanese Patent Application Laid-Open No. HEI 3-130705, a first optical fiber having a larger core diameter and a smaller relative refractive index difference and a second optical fiber having a smaller core diameter and a greater relative refractive index difference are fusion-spliced together, and thus fusion-splice portion is heat-treated at a predetermined temperature the reafter. In the optical fiber connecting method disclosed in Japanese Patent Application Laid-Open No. SHO 57-24906, on the other hand, the first optical fiber whose core region has a higher refractive index is heat-treated more strongly than the second optical fiber after fusion splice. Both of the methods intend to diffuse dopants in any of the first and second optical fibers upon the heat treatment, so as to lower the difference in their core diameters, thus making it possible to decrease the splice loss at the fusion-splice portion.
Using these conventional optical fiber connecting method is supposed to lower the splice loss at the fusion-splice portion between the above-mentioned single-mode optical fiber and dispersion-compensating optical fiber to about 0.3 to 0.6 dB. It is also supposed that the splice loss at the fusion-splice portion between the above-mentioned positive and negative dispersion optical fibers can be lowered to about 0.3 dB.
However, the splice loss at the fusion-splice portion has not yet been considered small enough although it is somewhat reduced by the conventional techniques disclosed in the above-mentioned two publications.
The inventors of the present invention observed the glass state near the fusion-splice portion in fusion-spliced two optical fibers in detail. As a result of the observation, it has been seen that, when a standard single-mode optical fiber and a dispersion-compensating optical fiber are fusion-spliced, the core region in the dispersion-compensating optical fiber deforms as the mode-field diameter is smaller.
Based on the inventors findings mentioned above, for eliminating the aforesaid problems, it is an object of the present invention to provide an optical transmission line constituted by optical fibers having structures different from each other in which the connection loss at their fusion-splice portion is further lowered.
The optical transmission line in accordance with the present invention is an optical transmission line including a portion formed by fusion-splicing optical fibers having structures different from each other; wherein, in the optical fibers having structures different from each other, a first optical fiber has a mode field diameter smaller than a mode field diameter of a second optical fiber fusion-spliced thereto; and wherein the average viscosity from the center to the outermost layer in the first optical fiber is greater than the average viscosity from the center to the outermost layer in the second optical fiber.
When the average viscosities in the first and second optical fibers are set as such, the deformation of the core region of the first optical fiber having a smaller mode field diameter becomes smaller upon fusion splice, whereby the splice loss can be restrained from increasing due to changes in fiber structures.
Preferably, after the first and second optical fibers are fusion-spliced, the optical transmission line is heat-treated at the highest heating temperature of at least 1300xc2x0 C. but not exceeding 1800xc2x0 C. within a range having a length of at least 1 mm but less than 10 mm centered at the fusion-splice portion. In this case, the splice loss can further be reduced.
The first optical fiber may be one having at least two cladding region layers surrounding a core region, and the average viscosity of the outermost cladding region layer greater than that of the core region. In this case, the cladding region does not deform upon fusion splice in the first optical fiber, so that the core region is restrained from deforming upon heating, whereby the splice loss can be kept from increasing. Preferably, the first optical fiber has a core region doped with GeO2 at a dopant concentration of at least 18 wt %, a first cladding region doped with F element, and an outermost cladding region layer doped with Cl element.
Preferably, the second optical fiber has at least one cladding region layer surrounding a core region, and the average viscosity of the outermost cladding region layer lower than any of the average viscosity of the core region and that of the outermost cladding region layer in the first optical fiber. In this case, no large structural changes occur in the core region in the second optical fiber even when its cladding softens upon fusion splice. Preferably, the second optical fiber has a core region doped with Cl element and a cladding region doped with F element. Alternatively, the second optical fiber may have two cladding region layers, the outer cladding region being doped with F element by an amount smaller than that in the inner cladding region.
Preferably, the core region of the second optical fiber has an outside diameter greater than the inside diameter of the outermost cladding region layer in the first optical fiber. In this case, the core region and first cladding region in the first optical fiber greatly influencing structural parameters thereof appear as if lidded with the core region of the second optical fiber, thus being surrounded with glass having a high viscosity, whereby their forms are easier to maintain.
Preferably, a part of the cladding region in the second optical fiber is doped with F element, whereas an outermost layer region thereof has an inside diameter of at least 1.05 times that of an outermost layer region in the first optical fiber.
A part of the cladding region of the second optical fiber may be doped with F element, the average viscosity of regions inside an outermost cladding region layer is greater than three times that of a region inside the outermost cladding region layer of the first optical fiber. Such setting can suppress the deformation of the region inside the outermost cladding region layer in the first optical fiber, whereby favorable connection characteristics can be obtained.
The first and second optical fibers may have unlike sign chromatic dispersions each other. Though the first and second optical fibers have mode field diameters greatly different from each other in general, the splice loss after fusion splice or after heat treatment can be made smaller in this case than in the conventional cases.